Nanocrystalline Alpha Alumina (&amp;#945;-Al2O3) and Method for Making the Same

ABSTRACT

A process for producing metastable nanocrystalline alpha-alumina (α-Al 2 O 3 ) having particle sizes smaller than 12 nm. Starting crystallites of α-Al 2 O 3  having a particle size larger than 12 nm, typically on the order of about 50 nm, are ball-milled at low temperatures to produce a nanocrystalline α-Al 2 O 3  powder having a particle size of less than 12 nm, i.e., below the theoretical room temperature thermodynamic size limit at which α-Al 2 O 3  changes phase to γ-Al 2 O 3 , wherein the powder remains in the α-Al 2 O 3  phase at all times.

CROSS-REFERENCE

This Application is a Nonprovisional of and claims the benefit ofpriority under 35 U.S.C. § 119 based on Provisional U.S. PatentApplication No. 62/410,990 filed on Oct. 21, 2016. The ProvisionalApplication and all references cited herein are hereby incorporated byreference into the present disclosure in their entirety.

TECHNICAL FIELD

The present disclosure relates to alpha-alumina (α-Al₂O₃), and inparticular to nanocrystalline α-Al₂O₃ and methods for making the same.

BACKGROUND

At the nanoscale, alumina (Al₂O₃) shows promise in many applications,ranging from high value areas such as cancer therapy and transparentarmor to more conventional areas such as polishing abrasives and cuttingtools because of its unique mechanical, optical, and electronicproperties. See H. Li et al., “Alpha-alumina nanoparticles induceefficient autophagy-dependent cross-presentation and potent antitumourresponse,” Nat Nano 6, 645-650 (2011); A. Krell et al., “TransparentSintered Corundum with High Hardness and Strength,” Journal of theAmerican Ceramic Society 86, 12-18 (2003); and H. Lei et al.,“Preparation of alumina/silica core-shell abrasives and their CMPbehavior,” Applied Surface Science 253, 8754-8761 (2007); see also U.S.Pat. No. 5,782,940 to P. S. Jayan et al., “Process for the preparationof alumina abrasives” (1998).

For example, alpha-alumina (α-Al₂O₃), more commonly known as corundum orsapphire, is one of the hardest known oxides behind only stishovite (ahigh pressure tetragonal SiO₂ phase) and boron sub-oxide (B₆O, an oxygendeficient metalloid). Dense nano-grained α-Al₂O₃ ceramics are theorizedto have hardness values substantially higher than those for singlecrystals while maintaining high in-line visible light transmission. SeeJ. A. Wollmershauser, et al., “An extended hardness limit in bulknanoceramics,” Acta Materialia 69, 9-16 (2014); M. A. Meyers,“Mechanical properties of nanocrystalline materials,” Progress inMaterials Science 51, 427-556 (2006); and R. Apetz, “TransparentAlumina: A Light-Scattering Model,” Journal of the American CeramicSociety 86, 480-486 (2003).

However, the crystal structure of alumina at the nanoscale depends onthe crystallite size of the nanoparticle; consequently, the mechanical,optical, and electronic properties can vary dramatically andnon-monotonically with particle size. See J. M. McHale et al., “SurfaceEnergies and Thermodynamic Phase Stability in Nanocrystalline Aluminas,”Science 277, 788 (1997); and A. H. Tavakoli et al., “Amorphous AluminaNanoparticles: Structure, Surface Energy, and Thermodynamic PhaseStability,” The Journal of Physical Chemistry C 117, 17123-17130 (2013).

The origin of the crystal structure versus crystallite size relationshipis found to be contingent on the surface energy of the crystalstructure. See McHale, supra. Therefore, the various synthesis routeswhich control the particle size result in the formed crystallitesadopting one of the various crystal structures of alumina (including γ,δ, χ, η, θ, κ, and α), each with their own set of symmetry-dependentproperties. See J. F. Nye, Physical Properties of Crystals: TheirRepresentation by Tensors and Matrices, pp. xv-xvi (St Edmundsbury PressLtd., 2004).

At the nanoscale, thermodynamics generally drives the crystal structureto alpha- (α-), gamma- (γ-), or amorphous alumina (a-Al₂O₃). Theoreticaland experimental studies show that α-Al₂O₃ has a significantly highersurface energy (2.64 J/m²) than γ-Al₂O₃ (1.67 J/m²) or a-Al₂O₃ (0.97J/m²). See A. Navrotsky, “Energetics of nanoparticle oxides: interplaybetween surface energy and polymorphism,” Geochem. Trans. 2003, 4(6),34-37.

FIGS. 1A and 1B are plots illustrating the relationship of specificsurface area (FIG. 1A) and particle size (FIG. 1B) to the thermodynamicstability of α-Al₂O₃, γ-Al₂O₃), and amorphous alumina (a-Al₂O₃).

Different surface energies are known to stabilize different polymorphs,see Navrotsky, supra, and, as particle size is reduced, the increase inspecific surface area relative to volume changes the relative energy offinite sized crystal particles. As illustrated in FIG. 1A, the highsurface energy of α-Al₂O₃ relative to γ-Al₂O₃ causes α-Al₂O₃ to becomethermodynamically unstable, i.e., to have higher excess enthalpy withrespect to γ-Al₂O₃ when the Al₂O₃ material has specific surface areaslarger than ˜100-130 m²/g, see McHale, supra, and Tavakoli, supra, suchthat particles having such surface areas undergo a phase change fromα-Al₂O₃ to γ-Al₂O₃, Similarly, the relatively high surface energy ofγ-Al₂O₃ causes it to become unstable with respect to a-Al₂O₃ at specificsurface areas larger than ˜370 m²/g (see FIG. 1A), producing a phasechange from γ-Al₂O₃ to a-Al₂O₃ in particles of that size. Since theratio of atomic surface to atomic volume increases as the particle sizeis reduced, these thermodynamic determinations point to size-dependenteffects at the nanoscale. Assuming a spherical particle shape whose sizeequals the crystal size, the surface area of Al₂O₃ nanoparticles largerthan ˜11-12 nm should cause such particles to have an alpha structure,while Al₂O₃ nanoparticles smaller than ˜3-5 nm will be amorphous (seeFIG. 1B). Alumina nanoparticles from 5-11 nm adopt the gamma structure.

However, synthesis of nanocrystalline Al₂O₃ typically results in γ-Al₂O₃when the crystallite size is ˜20-50 nm and α-Al₂O₃ when the crystal sizeis >50 nm.

Alpha-alumina particle sizes larger than those predicted by purethermodynamic calculations are likely the result of the fast coarseningkinetics of α-Al₂O₃. See McHale, supra. Bottom-up synthesis techniquesproduce small a-Al₂O₃ or γ-Al₂O₃ particles which are very stable, evenat high temperatures, due to the high energy barrier to α-Al₂O₃nucleation. See McHale, supra, and Tavakoli, supra. Converting suchparticles to α-Al₂O₃ requires high temperatures, often in excess of˜1000° C. (see G. P. Johnston et al., “Reactive Laser Ablation Synthesisof Nanosize Alumina Powder,” Journal of the American Ceramic Society 75,3293-3298 (1992); and S. Pu et al., “Disperse fine equiaxed alphaalumina nanoparticles with narrow size distribution synthesized byselective corrosion and coagulation separation,” Scientific Reports 5,11575 (2015)). However, α-Al₂O₃ is known to rapidly coarsen, i.e.,develop larger particle sizes, at temperatures above 500° C., seeMcHale, supra, so that the temperatures needed for phase transformationwill cause the small nanostructure to be lost. Thus, purely bottom-upsynthesis techniques will inevitably eventually produce α-Al₂O₃particles having crystallite sizes much larger than idealized intheoretical considerations.

Despite the difficulties in producing small nanocrystalline α-Al₂O₃,many works have claimed to find intricate and novel methods tosynthesize bulk amounts of small α-Al₂O₃ nanoparticles. Karagedov et al.synthesized ˜25 nm α-Al₂O₃ nanoparticles using a ball mill with anunidentified “grinding catalyst.” See G. R. Karagedov et al.,“Preparation and sintering of nanosized α-Al₂O₃ powder,” NanostructuredMaterials 11, 559-572 (1999). Yoo et al. used AlCl₃ with vapor phasehydrolysis to make an alumina precursors to be calcined into α-Al₂O₃that had particle sizes of about 35 nm. See Y. S. Yoo et al.,“Preparation of α-alumina nanoparticles via vapor-phase hydrolysis ofAlCl₃ ,” Materials Letters 63, 1844-1846 (2009). Borsella et al. usedlaser synthesis from gaseous precursors to synthesize 15-20 nm α-Al₂O₃.See Borsella et al., “Laser-driven synthesis of nanocrystalline aluminapowders from gas-phase precursors,” Applied Physics Letters 63,1345-1347 (1993). Zhang et al. calcined boehmite (γ-AlO(OH)) at 1000° C.to obtained rod like α-Al₂O₃ with sizes of 15 nm by 150 nm. See X. Zhanget al., “Nanocrystalline α-Al₂O₃ with novel morphology at 1000° C.,”Journal of Materials Chemistry 18, 2423-2425 (2008). Laine et al.utilized a liquid-feed flame spray pyrolysis to convert mixtures ofintermediate alumina phases into α-Al₂O₃ with the smallest size being 30nm. See R. M. Laine et al., “Nano α-Al₂O₃ by liquid-feed flame spraypyrolysis,” Nat Mater 5, 710-712 (2006). Das et al. used thermaldecomposition of an aqueous solution of aluminum nitrate and sucrosewhich was subsequently calcined to get a 20 nm sized particles. See R.N. Das et al., “Nanocrystalline α-Al₂O₃ Using Sucrose,” Journal of theAmerican Ceramic Society 84, 2421-2423 (2001).

However, these methods still produce α-Al₂O₃ larger than thethermodynamic limit of ˜12 nm, which reinforces the assumption that 12nm is a lower limit to the size of an α-Al₂O₃ nanoparticle.

A very recent work has demonstrated the possibility that Al₂O₃ can becoerced into the alpha structure below 12 nm. Pu et al. used a low yieldselective corrosion and refined fractionated coagulation separationtechnique to synthesize 10 nm α-Al₂O₃. See Pu, supra. The Pu techniqueused Fe to stabilize the surface energy of α-Al₂O₃ and precipitatedα-Al₂O₃ within a solid Fe grain. Though high nanoparticle yield and easeof industrial scale-up are not practical for such an approach, itdemonstrates the possibility of metastability of α-Al₂O₃ below thethermodynamic size limit.

However, all of these prior methods rely on expensive and uniquemachines that produce limited quantities of powder which hinders thosetechniques from leading the industrial production of nanocrystallineα-Al₂O₃ at or below the thermodynamics size limit.

SUMMARY

This summary is intended to introduce, in simplified form, a selectionof concepts that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter. Instead, it ismerely presented as a brief overview of the subject matter described andclaimed herein.

The present invention provides a process for producing metastablenanocrystalline alpha-alumina (α-Al₂O₃) having nanoparticle sizessmaller than 12 nm.

In accordance with the present invention, starting crystallites ofα-Al₂O₃ having a particle size larger than 12 nm, typically on the orderof about 50 nm, are ball-milled using a WC—Co media to produce ananocrystalline α-Al₂O₃ powder having a particle size of less than 12nm, i.e., below the theoretical room temperature thermodynamic sizelimit at which α-Al₂O₃ changes phase to γ-Al₂O₃, wherein the powderremains in the α-Al₂O₃ phase at all times.

The process in accordance with the present invention provides aneconomical, scalable industrial, scalable, and economical procedure thatprovides a new avenue for nanocrystalline α-Al₂O₃ processing. Thisprocessing route is unique and vitally important given that α-Al₂O₃ doesnot ordinarily exist at these crystallite sizes and that the currentbottom-up synthesis approaches can only produce γ-Al₂O₃, which is anundesired phase for many applications, at these small crystal sizes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are plots illustrating the thermodynamic instability(excess enthalpy) of the three alumina polymorphs as a function ofsurface energy (FIG. 1A) and particle size (FIG. 1B).

FIG. 2A AND 2B are plots of the X-ray diffraction patterns oflab-synthesized (FIG. 2A) and commercially prepared (FIG. 2B) α-Al₂O₃starting powder having a particle size of ˜50 nm, a ˜10 nm α-Al₂O₃powder produced from such starting powders after 270 minutes of ballmilling, and the ˜10 nm ball-milled α-Al₂O₃ powder after washing withnitric acid and hydrogen peroxide.

FIG. 3 is a plot showing grain size and WC—Co contamination of thelab-synthesized and commercially prepared α-Al₂O₃ powders as a functionof milling time.

FIG. 4 is a Transmission Electron Microscope (TEM) image of an α-Al₂O₃powder produced from 270 minutes of ball milling and washing inaccordance with the present invention on top of a holey carbon grid,showing that the grain size of the powder is well below 50 nm.

FIG. 5 is a High Resolution TEM image of the individual aluminacrystallites produced by a ball milling process in accordance with oneor more aspects of the present disclosure.

DETAILED DESCRIPTION

The aspects and features of the present invention summarized above canbe embodied in various forms. The following description shows, by way ofillustration, combinations and configurations in which the aspects andfeatures can be put into practice. It is understood that the describedaspects, features, and/or embodiments are merely examples, and that oneskilled in the art may utilize other aspects, features, and/orembodiments or make structural and functional modifications withoutdeparting from the scope of the present disclosure.

The present invention provides a process for producing metastablenanocrystalline alpha-alumina (α-Al₂O₃) having a nanoparticle sizesmaller than 12 nm.

The process of the present invention utilizes temperature-controlledhigh-energy ball milling and a simple acid washing technique to providea simple, scalable, industrial process for producing metastablenanocrystalline α-Al₂O₃ having a nanoparticle size at or below theroom-temperature thermodynamic α-Al₂O₃ particle size limit of ˜12 nm,i.e., below the size at which room-temperature thermodynamics woulddictate that the alumina changes from the α-phase to the γ-phase.

The general concept behind the present invention is that the α-Al₂O₃particles are ball-milled at a low processing temperature, typicallyroom temperature, where the low processing temperature kineticallyhinders any phase transformation of α-Al₂O₃ to γ-Al₂O₃ during theprocess because the activation barrier is too high.

Thus, in accordance with the present invention, a starting powder ofα-Al₂O₃ crystallites having a particle size larger than 12 nm, typicallyabout 50 nm, is placed into a ball-milling jar and is ball-milled atroom temperature using a ball-milling media until the startingcrystallites are reduced in size to a particle size of less than 12 nm.In most cases, the ball-milling media will be tungsten carbide-cobalt(WC—Co), though other containers and ball-milling media comprisingmaterials harder than α-Al₂O₃, such as diamond jars and media,diamond-coated jars and media, or cubic boron nitride jars and/or media,can also be used.

In most embodiments, the starting α-Al₂O₃ crystallites are ball milledin a series of short time intervals in order to prevent the temperatureof the α-Al₂O₃ crystals in the WC—Co ball-milling jar from increasing toa point where a phase change might occur. In a typical case, thestarting crystals are ball-milled for about 30 minutes at time, withabout 90 minutes being needed to reduce crystallites having a particlesize of ˜50 nm to crystallites having a size of less than 12 nm.However, one skilled in the art will readily appreciate that one or bothof the time intervals needed to prevent temperature increases of theα-Al₂O₃ crystals or the total time needed to obtain a powder having thedesired particle size may vary, depending on the volume of the WC—Cojar, the amount of the starting α-Al₂O₃ powder, or the amount of theWC—Co ball-milling media.

In other embodiments, the temperature of the α-Al₂O₃ can be kept down byusing active cooling measures, such as performing the ball milling atcryogenic temperatures, i.e., at or below about −180° C.

In all cases, in accordance with the present invention, the ball-millingwill be performed under a set of predetermined conditions configured toensure that the starting α-Al₂O₃ crystallites remain in a metastableα-Al₂O₃ phase at all times and do not change to the γ-phase at any timeduring the ball-milling process.

The sub-12 nm α-Al₂O₃ powders produced by the ball-milling process inaccordance with the present invention can then be washed with nitricacid and hydrogen peroxide to completely remove the WC—Co contaminationon the surfaces of the particles.

High Resolution TEM showed that the individual crystallites resultingfrom this ball milling process have differing crystallographicorientations with nominal sphericity, and are a truly nanocrystallinepowder.

The process in accordance with the present invention thus provides anindustrial, scalable, and economical procedure that provides a newavenue for processing of nanocrystalline α-Al₂O₃. The success providedby the process of the present invention in WC—Co ball milling of largestarting crystallites of α-Al₂O₃ down to metastable sub-12 nmcrystallites without those crystallites undergoing an undesirable phasechange was unexpected by the inventors and is contrary to what wouldnormally be expected from the thermodynamic characteristics of thevarious phases of alumina. The results provided by the process of thepresent invention are vitally important since α-Al₂O₃ does notordinarily exist at these crystallite sizes, and the current bottom-upsynthesis approaches can only produce γ-Al₂O₃, which is an undesiredphase for many applications, at these small crystal sizes.

Example

The process in accordance with the present invention was employed onlab-synthesized and commercially available α-Al₂O₃ powders todemonstrate the universal nature of the approach and to re-reinforce itsindustrial scalability.

Lab-synthesized α-Al₂O₃ nanopowders were synthesized via a modifiedreverse strike co-precipitation route similar to other systems reportedin the literature. See J. W. Drazin et al., “Phase Stability inNanocrystals: A Predictive Diagram for Yttria-Zirconia,” Journal of theAmerican Ceramic Society 98, 1377-1384 (2015); and J. W. Drazin et al.,“Phase Stability in Calcia-Doped Zirconia Nanocrystals,” Journal of theAmerican Ceramic Society 99, 1778-1785 (2016).

Aluminum nitrate nonhydrate (>98%, Sigma-Aldrich, St. Louis, Mo.) wasdissolved in de-ionized water to form a clear, dilute solution. Thecationic solution was then added drop-wise to a stirred 5M excessammonium hydroxide solution where aluminum hydroxide formed as a whiteprecipitate. The final mixture was washed with 190 proof denaturedethanol and centrifuged (5000 rpm for 4 min) 3 times to remove andreplace the excess ammonia solution. The white precipitate was thendried at 70° C. for 48 hours. The dried powder was heated to 1200° C.and held at temperature for 2 hours to calcine the hydroxide toα-alumina with a crystallite size of 50 nm.

It is noted that 50 nm was the smallest possible crystallite size thatwe were able to produce in the lab such that the powder was 100%α-Al₂O₃. Unfortunately, all attempts at seeding the synthesis with 50+nm α-Al₂O₃ powder (as reported by Li and Sun) were unable to lower thecalcining temperature or the crystallite size. See J. G. Li et al.,“Synthesis and sintering behavior of a nanocrystalline α-aluminapowder,” Acta Materialia 48, 3103-3112 (2000). Therefore, to test theefficacy of the ball-milling method of the present invention, theinventors decided to use a commercial α-Al₂O₃ powder having the smallestgrain size commercially available (99.9+% α-Al₂O₃ powder obtained fromStanford Advanced Materials, Irvine, Calif.) and ball mill thiscommercial powder to shear or grind the agglomerates and grains andcompare the results obtained to results obtained for the lab-synthesizedpowder.

The starting crystallite sizes for the lab-synthesized and commercialα-Al₂O₃ were 50 and 60 nm, respectively, as determined by diffractionpeak broadening analysis.

The α-Al₂O₃ powders were ball milled in a SPEX 8000M using a cobaltcemented tungsten carbide vial in 3 g batches. The milling mediaconsisted of four 6 mm and two 12 mm tungsten carbide (WC) balls (WC—Co,McMaster-Carr, Elmhurst, Ill.) giving a ball to powder mass ratio (BMR)was ˜10:1. The batches were milled in 30 minutes intervals andre-oriented at each interval to ensure that the powder did notpreferentially collect inside the vial during milling. The crystallitesize of the lab-synthesized and commercial powders after milling wasabout 9.6 nm and 8.7 nm, respectively.

After ball milling, the powder was washed using a 5% (v/v) 15.4M HNO₃ in30% H₂O₂ solution to dissolve the residual tungsten carbide-cobalt(WC—Co) contamination from the ball-milled powder in a manner similar tothat done by Archer et al. in their previous work. See M. Archer et al.,“Analysis of cobalt, tantalum, titanium, vanadium and chromium intungsten carbide by inductively coupled plasma-optical emissionspectrometry,” Journal of Analytical Atomic Spectrometry 18, 1493-1496(2003). The dissolution of the residual WC—Co contamination wasperformed at 95° C. while under magnetic stirring and was finished inunder an hour. The mixture was then washed with 190-proof denaturedethanol 3 times.

X-ray diffraction (XRD) analysis of the resulting powders was performedon a 18 kW rotating anode Rigaku X-Ray Diffractometer (Rigaku, Tokyo,Japan) operated at 50 kV and 200 mA using a copper target. Thecrystallite size was calculated using JADE 9.6 v software to perform awhole pattern fitting (WPF) refinement using PDF#10-0173 for α-Al₂O₃ andPDF#51-0939 for WC—Co (when presenting the diffraction pattern).Specific surface area measurements were made on an ASAP 2020 sorptionapparatus (Mircomeritics, Norcrosss, Ga.) using a five point liquidnitrogen adsorption measurement (BET theory). The reported values are anaverage of three consecutive measurements.

The results of the XRD analysis of the two α-Al₂O₃ samples after ballmilling for 270 minutes are shown in FIGS. 2A and 2B.

As can be seen from the plots in FIGS. 2A and 2B, both the milled thelab-synthesized and commercial powders retained their α-structure eventhough their crystallite size (as determined by the width of the peaksin the XRD analysis using a Halder-Wagner analysis) was reduced belowthe thermodynamic phase-crossover size limit of 9.6 nm and 8.7 nm forthe lab-synthesized (FIG. 2A) and commercial powders (FIG. 2B),respectively.

The powders, which were white before the milling, developed a greyishcolor after milling as a result of the WC—Co milling media wearingagainst and so contaminating the surface of the nanopowder. Thiscontamination is also observable in the diffraction pattern. Forexample, the major peak seen at 31.8° in FIG. 2B is associated with thisWC contamination.

In order to remove the WC contamination, a modified washing procedurewas employed using nitric acid and hydrogen peroxide. The hydrogenperoxide oxidized the WC such that the product was dissolvable in nitricacid. See Archer, supra. With the low concentrations of cobalt cementused in this example, nitric acid was sufficient to prevent a cobaltpassivation layer from forming; however, aqua regia, instead of nitricacid, can also be employed where appropriate to remove excess cobaltmedia from the milled nanopowder.

After this acid washing, the powders returned to their bright whitecolor. The XRD of the powders after acid washing are shown by the dottedlines in FIGS. 2A and 2B, where there was no phase or grain size changein the powders. The elimination of the WC contamination peak and thereturn of the powder to a bright white color signify that the washingwas sufficient to remove the WC—Co contaminate.

The plots in FIG. 3 show that there is a practical limit to the minimumobtainable crystallite size. The time-dependency of the crystallite size(solid lines) and the WC contamination levels (dotted lines) of the twopowders show that the crystallite size of both powders followed verysimilar, almost overlapping, exponential decaying functions plateauingnear 10 nm.

The similarity in trends of the two powders suggests that the techniqueof the present invention may not be well suited to produce nanopowdersmuch smaller than 9 nm. The plateau in the final sub-10 nm crystallizesize can be explained by a Hall-Petchian-type phenomenon: as the size ofthe powder decreases, the hardness of the individual crystals andagglomerates increases to a point where it is significantly harder thanthe milling media, wearing the WC balls in the ball-milling chamberfurther. See D. Chrobak et al., “Deconfinement leads to changes in thenanoscale plasticity of silicon,” Nat Nano 6, 480-484 (2011); and Y.Tian et al., “Ultrahard nanotwinned cubic boron nitride,” Nature 493,385-388 (2013).

Consequently, there is an equilibrium where the α-Al₂O₃ powder will beas hard as or harder than the milling media such that continued millingwill not significantly alter the grain size. This assumption issupported by the WC contamination curve in FIG. 3, which is essentiallyzero for the lab-synthesized powder produced in accordance with thepresent invention until minute 175, but then increases rapidly while thegrain size barely decreases.

Interestingly, the commercial α-Al₂O₃ powder accumulated WCcontamination at an earlier milling time compared to the lab-synthesizedpowder having similar grain sizes. One reason for this difference incontamination levels could be the agglomeration state of the powders:i.e., the amount of free surfaces in the powder.

Brunauer-Emmett-Teller (BET) surface area analysis showed that thelab-synthesized and commercial powder, after milling and washing, had asurface area per gram of 62.101±0.217 and 42.983±0.120 m²·g⁻¹respectively. Consequently, even if the crystallite size plateau of 10nm is an artifact of the milling time, the amount of WC contaminationwould grow too fast to produce significantly smaller grain sizes.Regardless, this is the first time that a simple procedure, with highyield, has demonstrated the feasibility of producing α-Al₂O₃ with highspecific surface areas and crystallite sizes at or below the γ-phasethermodynamic crossover limit.

In another examination of the metastable α-Al₂O₃ nanopowders produced inaccordance with the method of the present invention, TEM samples wereprepared by dispersing the nanopowders in methonal and then pipettingthe nanopowder dispersion onto a holey carbon grid. All specimens wereexamined in a FEI Tecnai G2 TEM with LaB6 filament operated at 300 kV.

FIG. 4 shows a TEM micrograph of an α-Al₂O₃ nanopowder produced fromlab-synthesized α-Al₂O₃ starting powder, after ball-milling for about270 minutes at approximately 30-minute intervals and removal ofcontaminants. The crystallite size of this powder is less than 10 nm,consistent with the reported XRD grain size. In addition, as can beeasily seen from the image in FIG. 4, the powder exhibits a high levelof agglomeration, which would be expected for sub-10 nm size particlesbecause of their high electrostatic attraction to one another.

FIG. 5 is a high-resolution TEM image of the powder depicted in FIG. 4,and reveals that the small grains shown in FIG. 4 have uniquecrystallographic orientations and are distinct individual particles. Theobserved crystallite size of the particles, on the order of 10 nm,matches well with the XRD grain size. The image further suggests thatthe crystals are not related by low angle misorientations, furthershowing that the produced powder has a sub-10 nm grain size.

Thus, although it is not presently feasible to directly synthesizenanocrystalline α-Al₂O₃ having a sub-12 nm grain size, the presentinvention provides a method for producing a metastable α-Al₂O₃nanocrystalline powder having a crystallite size of less than 12 nmusing a novel WC—Co ball milling and nitric acid and hydrogen peroxidewashing procedure. The procedure utilizes high-energy room-temperatureball milling that is conducted at a series of short intervals in orderto avoid the α- to γ-phase transition despite the final grain size beingbelow the thermodynamic size limit for the α-phase, while the washingeffectively removed the milling contamination from the final powders.The procedure worked equally well for the lab-synthesized powders as forthe commercial powders, producing similar crystallite sizes withsufficiently high yields that the overall procedure could be scaled upwith minimal modifications. Therefore, this novel technique is the firstto provide a pathway for the industrial production of metastablenanocrystalline α-Al₂O₃ below the thermodynamic size limit.

Although particular embodiments, aspects, and features have beendescribed and illustrated, one skilled in the art would readilyappreciate that the invention described herein is not limited to onlythose embodiments, aspects, and features but also contemplates any andall modifications and alternative embodiments that are within the spiritand scope of the underlying invention described and claimed herein. Thepresent application contemplates any and all modifications within thespirit and scope of the underlying invention described and claimedherein, and all such modifications and alternative embodiments aredeemed to be within the scope and spirit of the present disclosure.

What is claimed is:
 1. A process for producing a metastable α-Al₂O₃nanocrystalline powder having a particle size of less than 12 nm,comprising: placing starting crystallites of α-Al₂O₃ having a particlesize larger than 12 nm into a ball-milling jar containing a ball-millingmedia; and ball-milling the α-Al₂O₃ starting crystallites to produce ametastable ball-milled α-Al₂O₃ nanocrystalline powder having a particlesize of less than about 12 nm; wherein the ball-milling is performed ina plurality of short time intervals, a length of each time interval anda total time of ball-milling being configured to prevent a temperatureof the starting α-Al₂O₃ from increasing to a point at which α-Al₂O₃undergoes a phase change to γ-Al₂O₃; wherein the α-Al₂O₃ startingcrystallites and the ball-milled α-Al₂O₃ nanocrystalline powder remainin the α-Al₂O₃ phase at all times.
 2. The process according to claim 1,wherein the ball-milling media is WC—Co.
 3. The process according toclaim 1, wherein the ball-milling is conducted in a series of 30-minutetime intervals.
 4. The process according to claim 1, further comprisingwashing the ball-milled α-Al₂O₃ nanopowders with nitric acid andhydrogen peroxide.
 5. A process for producing a metastable α-Al₂O₃nanocrystalline powder having a particle size of less than 12 nm,comprising: placing starting crystallites of α-Al₂O₃ having a particlesize larger than 12 nm into a ball-milling jar containing a ball-millingmedia; and ball-milling the α-Al₂O₃ starting crystallites to produce ametastable ball-milled α-Al₂O₃ nanocrystalline powder having a particlesize of less than about 12 nm; wherein the ball-milling is at a lowtemperature configured to prevent a temperature of the starting α-Al₂O₃from increasing to a point at which α-Al₂O₃ undergoes a phase change toγ-Al₂O₃; wherein the α-Al₂O₃ starting crystallites and the ball-milledα-Al₂O₃ nanocrystalline powder remain in the α-Al₂O₃ phase at all times.6. The process according to claim 5, wherein the ball-milling media isWC—Co.
 7. The process according to claim 5, wherein the ball-milling isperformed at temperatures at or below about −180° C.
 8. The processaccording to claim 5, further comprising washing the ball-milled α-Al₂O₃nanopowders with nitric acid and hydrogen peroxide.